Pulleys are used for raising and lowering objects and/or people. In particular, pulleys may be used for lowering equipment or rescue workers from helicopters and lowering equipment or workers down shafts. A typical standard pulley comprises a wheel or sheave for carrying a rope, the sheave being rotatably mounted on an axle or the like which turns in a frame or block. Pulling at one end of the line (the haul end), will raise the weight or object attached at the other end of the line. Standard pulleys run very freely. When very heavy loads are being lowered, such free running of the pulley is disadvantageous, as the operator on the haul end of the line may have little control over the object at the other end of the line, particularly if the object is heavy or if the object accelerates and starts to descend rapidly. This control problem can be overcome by fixing the wheel/sheave, so that it is not able to turn and the rope merely slides over the surface of the sheave with the resultant increase in friction providing a greater degree of control for the operator. The sheave may comprise an elongate drum so that the line may be wrapped around the drum several times to increase the friction and consequently, increase the ease of control.
The problem with this approach, however, is that although a greater degree of control is provided when lowering an object, the increase in friction makes it more difficult to raise an object using fixed pulley. To solve this problem, it is known to provide one-way pulleys where the drum will turn in one direction only. Such pulleys can be used as either static pulleys or standard (free running) pulleys.
A further problem which arises with pulleys, is the tendency for the ropes to overrun which occurs when the ropes that are wrapped around the drum rub against each other and ride up over each other. This problem not only increases wear and tear on the rope, but may also reduce the control the operator has at the haul end of the line over the object being raised or lowered. It is known to put one or more pins at the top of static pulleys act as a rope guide. It is also known to mount a blade adjacent the top of a standard pulley to guide the rope and prevent screwing of the rope on the drum. The problem with this is that it limits the number of turns of rope that may be wound round the drum to one turn, where one blade is provided.
A yet further problem with existing pulleys, is that while pulleys are useful for lowering objects, a lack of mechanical advantage in existing pulleys means that it is usually difficult to raise and lower an object or person, except by using a lower pulley having two or more sheaves below the main pulley. However the problem with this is that more rope is needed passing between the main pulley and sheaves of the lower pulley.
The present invention seeks to alleviate some of the above mentioned problems of existing pulleys.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.